1. Technical Field
The invention relates to the art of electrochemical cells and more particularly, to a corrosion-resistant and durable metal bipolar plate capable of functioning in the highly corrosive fuel cell environment and a method of manufacturing the same.
2. Background of Related Art
Due to increasing demand in the earth's limited energy resources and low conversion efficiencies of conventional power generation systems as well as environmental concerns, the need for a clean, reliable, and renewable source of energy has greatly escalated. Fuel cells are one of the most promising techniques to meet this need.
In a fuel cell, the chemical energy is provided by a fuel, such as hydrogen, and an oxidant, such as oxygen, stored outside the fuel cell. Functionally, a fuel cell has two electrodes flanking an electrolyte. Oxygen passes over one electrode and hydrogen over the other, generating electricity, water and heat.
Among numerous types of fuel cells, a proton exchange membrane cell (PEM) type is known for operating at relatively low temperatures (about 200° F.), as well as for having high power density and varying their output quickly to meet shifts in power demands. All of the above-mentioned characteristics are found particularly attractive to automobile industry, where PEMs have been declared “the primary candidate for high-duty vehicles.”
The heart of the fuel cell, including the PEM fuel cell, is a thin, solid polymer membrane-electrolyte having an anode on one face of the membrane-electrolyte, whereas the other face thereof is provided with the cathode. The membrane is sandwiched between a pair of electrically conductive contact elements which serve as a current collectors configured to deliver and distribute the fuel cell's gaseous reactants (H2 and O2/air) over the surfaces of the respective anode and cathode.
A bipolar PEM fuel cell comprises a plurality of the membrane-electrode-assemblies stacked together in the electrical series while being separated one from one another by an impermeable, electrically conductive contact element known as a bipolar plate. The opposite faces of the bipolar plate are juxtaposed with the anode of one cell and the cathode of the other cell, respectively. Accordingly, the bipolar plate separates adjacent cells and electrically conducts current therebetween. Furthermore, the bipolar plates give rigidity to the PEM fuel cell stack and support the membranes. In addition, the various flow channels for air, fuel and coolant are typically incorporated into the bipolar plates.
In a working PEM environment, the bipolar plates are in constant contact with highly acidic solutions. Moreover, the cathode operates in.a highly oxidizing environment while being exposed to pressurized air. Also, the anode is constantly exposed to super atmospheric hydrogen and an acidic environment as well. Hence, bipolar plates must be resistant to acids, oxidation, hydrogen and brittleness. Otherwise, the bipolar plates are affected by corrosion, which is detrimental to the performance of fuel cell as it fouls the catalyst of the electrode in the membrane electrode assembly and steadily degrades the cell's power output.
Graphite is currently the most popular bipolar material for fuel cell applications because of its non-corrosive property. However, graphite is brittle and porous making it extremely difficult to machine and assemble bipolar plates during the production process. Still another consequence stemming from the inherent characteristics of graphite is that it is a poor load-support structure. Note that for the operation of the fuel cell, it is necessary to generate two perfectly sealed chambers, one for oxygen and the other for hydrogen. Since high loads are detrimental to structural integrity of graphite, a fuel cell stack often leaks externally because of the lack of sufficient forces capable of reliably sealing adjacent bipolar graphite plates. In addition, the fuel cell stack leaks internally as a result of the porosity of graphite. To combat this problem, the surfaces of the graphite bipolar plates are covered by a sealant, which, unfortunately, decreases the electrical conductivity of the graphite surfaces and, thus, increases the heat loss. Both types of leakage represent a safety hazard (hydrogen is extremely explosive) and may jeopardize the operation of the fuel cell. Finally, graphite is relatively expensive.
As an obvious alternative to graphite, lightweight metals such as aluminum and titanium and their alloys have been proposed. While metals are non-porous, highly electro-conductive, inexpensive, durable and have low density, their use in the PEM environment is limited because of their low resistance to corrosion.
Covering metal substrates with polymeric material increases electrical resistance of the bipolar plates. Hence, electrical conductivity of the bipolar plate at the very least decreases and, in some cases, may be dramatically reduced.
It is, therefore, desirable to produce reliable and efficient metal-based bipolar plates capable of exhibiting high resistance to corrosion in an acidic environment.